analysis of implant breakage in trochanteric fractures

$: Analysis of implant breakage in trochanteric fractures$
Journal of Optoelectronics and Biomedical Materials Vol. 1, Issue 2, June 2009, p. 172-179

Analysis of implant breakage in trochanteric fractures

C. DEMIAN*, V.-A. ŞERBAN, A. RĂDUŢĂ, I. VIDA-SIMITIa, R. PREJBEANUb, C. LOCOVEI “Politehnica” University of Timişoara, Faculty of Mechanics aThe Techincal University of Cluj-Napoca, the Faculty of Materials Science and Engineering b“Victor Babes” University of Medicine and Pharmacy, Timisoara The paper analysis the failure mechanism by breakage of a stainless steel AISI 316Ti plate, currently used for trochanteric fractures. The plate was removed from the body of a patient and then was subjected to tests. The implant was analyzed by optical and electronical microscopy and reveals a higher content of inclusions that lead to a severe form of corrosion in the area of screws for fastening, but also on free surfaces. The results obtained through finite elements analysis reveal that the frictional stresses and slipping tendencies at the interface implant- superior part of femur reach a maximum value in areas where fracture has occurred. (Received May 15, 2009; accepted May 26, 2009) Keywords: Implant, Trochanteric fracture, Crack, Nonmetallic inclusions, Fatigue facture 1. Introduction Recovering and analysing the implants offer critical informations that lead to a better

understanding of phenomenae on the implant-bone interface. Direct bone-metal osteointegration of the implants, in the form of stainless steel rod, was possible only due to the carefully controlled surgical techniques and the careful preparation of the surface of the implants [1], [2]. In order to facilitate the bone growth, porous layers over implants can be applied; relation between the sizes of pores and bone growth were obtained by studying recovered implants [3].

Some of the models of the osteosynthesis plates, used for different fractures, reveal significant corrosion, attributed initially to the use of a combination of metals (for example: AISI 316L fixed with AISI 304L screws) and later both to micro-galvanic corrosions and corrosion through micro-movements and friction between the components [4], [5], [6], [7].

2. Material and Methods The mechanism of corrosion was studied on a recovered failure implant used for

osteosynthesis of trochanteric fractures as 130° Angled Blade Plate, (DCP), made from austenitic stainless steel (Fig. 1).

Fig. 1. Osteosynthesis plate for trochanteric fractures


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Chemical composition of implant material was determined with optical emission spectrometer (ARL QuantoDesk Spectrometer).

Implant was metallographically prepared in a transversal sections, with the purpose of examination through optic metallographic microscope Olympus BX51M.

Before extracting the sample and microscopic investigation, aspect of corroded surfaces was investigated by a stereo Olympus SZX7 microscope.

As a means to quantify the maximum tolerable levels of stresses and strains of the implant-bone assembly a finite element analysis was carried out. The implant-bone assembly was subjected to loads and constraints similar to those found in human body. The mechanism of fatigue fracture was analyzed using the AFGROW program.

3. Results 3.1 Examination of fractured angled blade plate through optic and electronic microscope Table 1 shows experimentally determined chemical composition for angled blade plate,

while table 2 shows equivalent chemical composition corresponding to ASTM standards (AISI 316Ti).

Table 1. Composition of analysed implant material.

Element Concentration (%) Element Concentration (%) Fe 68.20 Ni 10.01 C 0.0435 Mo 2.428 Si 0.3622 Cu 0.1865

Mn 1.476 Ti 0.3591 P >0.046 Nb <0.003 S 0.0104 Co 0.1358 Cr 16.64

Chemical composition of tested alloy falls in the category of Stainless steel AISI 316Ti, according to ASTM A 240-00.

The fracture surface presents the aspect of a fatigue fracture, while the crack propagation was present in two zones (Fig. 2 a, b and c).

C. Demian, V.-A. Şerban, A. Răduţă, I. Vida-Simiti, R. Prejbeanu, C. Locovei

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a) OM 10x

b) OM 16x c) OM 16x

Fig. 2. The aspect of blade plate surface fracture

Microstructures of the tested sample from different areas, after the metallographic

preparation and chemical etching (Fig. 3) confirm the existence of some orientated series of nonmetallic inclusions, due to the plastic deformation.

a) OM 100x b) OM 500x

c) OM 500x d) OM 1000x

Fig. 3. Microstructures corresponding to plate used in osteosynthesis of trochanteric fractures implants

Initial crack – zone 1 Initial crack

– zone 2


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The examined material has a high content of inclusions as titanium nitrides, carbonitrides and chemical compounds as silicates, which lead to a severe form of corrosion in the area of screws for fastening, but also on free surfaces. It is presumed that cyclic loads, favored by corrosion (fatigue corrosion) have produced the fracture.

3.2 The finite elements analysis The finite element analysis was performed using ANSYS 10 software. A femoral model

formed of cortical and spongy bone structure, filled with marrow was taken in consideration [7], [8]. The material properties (ASISI 316Ti stainless steel) of fracture implant and the properties of the cortical and spongy bones and the marrow are presented in the Table 2.

Fig. 5. The equivalent stresses from implant, in the case of reduce friction between the fractured bones

Table 2. The mechanical and physical properties considered in the FEM analyses .

Value No. Properties AISI 316Ti stainless

steel Cortical

bone Spongy bone Marrow

1 Young modulus 1,96×1011 Pa 1,37×1010 Pa 3.89×108 Pa 1,1×109 Pa

2 Poisson ratio 0,39 0.3 0.28 0.42 3 Density 7900 kg/m³ 1,7 g/cm³ 500 kg/m³ 950 kg/m³

4 Yield tensile strength 2.00×108 Pa 1,2×108 Pa 7.36×106 Pa 2,5×107

Pa 5 Tensile strength 5.50×108 Pa 0,9×108 Pa

The finite element analysis were made using the following assumptions: the weight of the

patient is 80 Kg (G); during walking the value of the resultant force (R) is 3100 N, calculated by equation R = (4...4,5)·G, the components of resultant force are Rx = -1060,13 N; Ry = 0; Rz = 2913,1 N.

The discreet element mesh, having 25785 nodes and 14857 elements is presented in Fig. 4 a. Figure 4 b presents the model with highest loads applied, the connection between components being of bounded type, which means that nodes stay in contact during the deformation of the model subjected to loads [9].


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a) mashed model b) force applied on structure

Fig. 4. Presentation of initial data for finite elements analysis

The Von Mises stresses, that appear in the fixed implant in the case of reduce friction between the fractured bones are presented in Figure 5. This means there is a possibility of relative motion into the contact zone and represents the initial case, when the fractured bones were not fitted by callus increase. The fractured bones were fitted merely by implant.

The Von Mises stresses that appear in the fixed implant in the case of the fractured bones are beginning to fit by callus are presented in the Figure 6. The contact from the fracture zone is “no separation” type. This mean that the contact permits micromotions between the finite elements nods in the contact zone, without a relative motion of the fractured parts bone.

Fig. 6. The equivalent stresses from implant, in the case of the fractured bones fitted by callus The stresses from implant decrease as the friction between the fractured parts increase.

The maximum load values are positioned near the fatigue fractured zone. 3.3 The fatigue fracture analyses The damage of the angled blade plate is due to the cyclic loads that were amplified by the

action of corrosion (fatigue corrosion). These lead to brakeage and require the analyses of the fatigue fracture.

Fracture analyses of the recovered angled blade plate were realized using NASGRO equation numerically solved using AFGROW software. The recovered angled blade plate fracture is due to material fatigue. The goal of the fracture analysis is the numerical calculus of the cycles or of the number of steps made by the patient, until the plate fractures. The level of stresses used in NASGRO equation were the ones previously calculated by means of finite element analysis [10]. The method used by AFGROW is closed form solution, in this particular case, classic model of single edge corner crack has been used. Two identical semielliptical initial cracks were taken into


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consideration in order to calculate the number of cycles that lead to the fracture. The analysis were performed using initial crack depth, a, equals to 0.1 mm and initial crack length, c, equals to 1 mm. Such cracks can be easily produced by scratching during the implantation process and are enlarged by the natural corrosion due to the body fluids.

For a given material the crack growth rate, da/dN, is a complex function of many variables including stress intensity range (ΔK = Kmax - Kmin), stress ratio (R = Kmin / Kmax), temperature and frequency of the applied cyclic load. In the most general form it is simply written as:

,...),,,( fTRKfdNda

Δ= (1)

Where: da/dN – crack depth growth rate; ΔK – stress intensity factor range (ΔK = Kmax - Kmin);

A number of explicit equation forms exist to cater for simplified situations e.g. the Paris (equation 2) and Walker (equation 3).

nKCdNda )(Δ⋅= (2)

[ ]nmRKCdNda 1

0 )1( −−Δ= (3)

Where: C, n – fit parameters in the NASGRO equation; m – shape parameter of the Weibull distribution of the fracture resistance.

For the most general approach, the evaluation of da/dN must allow input of as many of the contributing factors as possible. An advanced approach is the so-called NASGRO expression (also called Forman–Newman–de Koning equation) jointly introduced by National Aeronautics and Space Administration (NASA) and European Space Agency (ESA), which is now common in aerospace applications [10], [11], [12]:

q

Jc

p

effneff

KK

KK

KCdNda

⎥⎦

⎤⎢⎣

⎡−

⎥⎥⎦

⎤

⎢⎢⎣

⎡

ΔΔ

−

⋅Δ⋅=max

0

1

1

)( (4)

Where: ΔKeff – effective stress intensity factor range (ΔKeff = Kmax - Kop, Kop – opening stress intensity factor, above which the crack is open); ΔK0 – threshold stress intensity factor range; KJc – fracture resistance of the material written in the terms of K;

p, q – fit parameters in the NASGRO equation. The fatigue fracture analysis was completed using the following geometrical conditions and material properties: crack depth a= 0.1 mm, crack length c = 1 mm, plate thickness T = 2 mm, plate width W = 10 mm and material properties were those for the wrought AISI 316Ti as in Table 3.


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Table 3. Mechanical and physical properties of the material

Material properties AISI 316Ti

Coefficient of Thermal Expansion 1,26x10-5

Effective fracture toughness for surface/elliptically shaped crack, KV at the 37 oC, J

153,838

Yield stress, Rp0,2, MPa 689,476 Paris crack growth rate constant 7,6575x10-12 Plane stress/strain constraint factor 2,5

The propagation curves a and c versus number of cycles are presented in the figure 7.

Fig. 7. The evolution of the cracks propagation versus number of cycles

The angled blade plate fractures after approximately 1.000.000 cycles that explain the

fact that the plate life span is about 1 year assuming daily load of the plate cumulate with material defects and surface scratches or microcracks.

4. Conclusion The aspect of fracture surface and microstructure of the material reveals the presence of

some material defects (non metallic inclusions) that increases the probability of fatigue fracture of the implant and lead to a higher rate of material corrosion.

The values obtained after simulation confirms this hypothesis. The maximum value of calculated stress is 160.5 MPa and does not reach the level 200-250 MPa that corresponding to yield tensile strength of AISI 316Ti stainless steel.

Acknowledgements All the investigations were performed in the Research Center for the Processing and

Characterisation of the Advanced Materials of the “Politehnica” University of Timisoara.


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The research was performed in the framework of COST ACTION 537 Core Laboratories for the Improvement of Medical devices in Clinical Practice from the Failure of the Explanted Prostheses Analysis (FEPA).

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analysis of implant breakage in trochanteric fractures

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